The invention is directed to apparatus and method for improving the efficiency of nonlinear optical process where the phase matching of the light beams passing through the nonlinear material, cannot be achieved by conventional means. This invention effectivly phase matches partially phased or non-phased matched beams by periodically removing the output energy of the beams after mixing within the nonlinear material.
A large body of literature exists on nonlinear processes and phase matching techniques employed to phase match light beams passing through nonlinear material. None of these techniques teach phase matching by removal of the output beam.
A key requirement for efficient nonlinear operation in devices of this type is that the beams be phased matched. Phase matching involves setting up the operating conditions so that the propagation vector of the output beam is equal to the sum of the propagation vectors of the input beams, ie. the, output beam is equal to the sum of the propagation vectors of the input beams, ie. ##EQU1## .lambda..sub.o' .lambda.j =vacuum wavelengths of output and input beams respectfully
n.sub.o' n.sub.j =indices of refraction for output and input beams respectfully PA1 N=number of input beams PA1 s.sub.o' s.sub.j =unit vectors in the propagation direction
Typically, phase matching is achieved by selecting the material, operating wavelengths and propagating directions correctly. The problem is well understood and detailed descriptions of phase matching techniques can be found in the technical literature e.g., in Applied Nonlinear Optics by Fritz Zernike and John E. Midwinter, chapter 3.
An important concept to understand about phase matching is that the waves interact constructively over a length l.sub.c called the coherence length. After propagating a length l.sub.c the waves interfere destructively for a length l.sub.c and the process repeats. The distance l.sub.c is equal to: ##EQU2##
For a crystal of length L&gt;&gt;l.sub.c the total nonlinear output appears to come from a region only l.sub.c long and the material is used very inefficiently.
Another important characteristic of the state of the art nonlinear processes is that many of the materials with the largest nonlinear coefficient cannot be phased matched because there is no combination of n.sub.o, n.sub.j, .lambda..sub.o, .lambda..sub.j, s.sub.o, and s.sub.j that leads to .DELTA.k=0.
U.S. Pat. No. 3,983,406, by inventors Lax et al., describes a technique to improve the efficiency of a nonlinear process by using multiple internal reflections. The converted signal and the reference signals all travel through the nonlinear medium where some adverse reverse mixing occurs and the reference signals and the converted signal are not separated until they leave the nonlinear medium.
The present invention improves the present state of the art by more efficiently separating the input beams and the resulting output beam from each other, thus preventing distructive interference mixing thereof without concern to the phase relationship of the input and output beams.